How do we know the college code of the college we r sending it to? Do they give us a book with the college codes? Also, do we have to write this college code on every exam we take, or just the first one

If you register online for the SAT they tell you the college code. You go "send score" and "search for college" and select the ones you need. When you print out your ticket the codes are on there as well. If you didn't register online... I suppose you can check on the college's website. Usually they have a page saying "The College Code for Uni of **** is 1234".